1. Field of the Invention
This invention pertains to synchronous shifting multi-range hydromechanical steering transmissions for tracklaying or skid-steered wheeled vehicles having hydraulic components to provide continuously variable hydrostatic drive ratios and mechanical components wherein selective operation of clutches or brakes causes the transmission to operate in a plurality of distinct hydrostatic, hydromechanical or mechanical ranges and wherein steering is effected by using one or more hydrostatic devices to impose a speed difference between left and right outputs. More specifically, this invention pertains to such transmissions having internal integral steering resulting from use of the same hydrostatic components for both drive and steer rather than from use of dedicated hydrostatic components added solely to impose a difference of speed to left and right outputs to effect steering (often referred to as "steerover"). In particular, this invention pertains to that class of multiple range steering transmission which also have distinctly different speed and torque ratios in the various ranges without having steer reversal, i.e., with "steer positive" steering and without having clutches in the steer path. In this respect, the speed and torque ratios referred to are understood to be those ratios existing between the output from the included hydrostatic component and the transmission output with the speed and torque ratios being related as inverse functions. That measurement is made between the hydrostatic and final outputs, especially in a split output transmission, because the ratios are fixed between transmission input and the input to the hydrostatic component and infinite between the input and the output of the hydrostatic component, i.e., within, or across, the hydrostatic component. A split output hydromechanical transmission, as the terminology is established in the art (Tuck, "Split Power Transmission", U.S. Pat. No. 3,433,095), is one in which a plurality of power paths from input to output are joined by a final power combining unit. In the split output example, the combining unit receives power in one path via a mechanical connection to transmission input and in another path from input via an included hydrostatic unit as opposed to a split input (Polak, U.S. Pat. No. 3,982,448).
2. Description of Prior Art
The transmission of Polak, U.S. Pat. No. 3,596,535, is an example of synchronous shifting multi-range hydromechanical steering transmissions using dedicated hydrostatic components for steering, i.e., a "steer-over" transmission. Polak is of particular interest because the different speed and torque ratios in different ranges are shown by the slope of the lines in FIG. 2 of that patent. The transmission of Tuck, U.S. Pat. No. 3,590,658, is one example of synchronous shifting multi-range hydromechanical steering transmissions using internal steer. However, this transmission is really two coordinated truck (single output) transmissions which have clutches in the steer path and require coordination of range shifts which is in turn complicated by the fact of steer reversal. The transmission of Polak, U.S. Pat. No. 3,538,790, is another example of a synchronous shifting multi-range hydromechanical steering transmission having integral or internal steer. It is full reversing, has a hydrostatic low range and a hydromechanical high range and has two mirror-image halves (i.e., two joined truck transmissions). The transmission disclosed in U.S. Pat. No. 3,815,698 (Reed) is a more pertinent example of a synchronous shifting multi-range hydromechanical steering transmission having positive internal steering. That transmission of Reed which is a predecessor model to the transmission from which the subject matter of this Application was derived has a hydrostatic combined reverse and first forward range and hydromechanical second and third forward ranges in which the speed and torque ratios, as defined above with respect to the field of invention, of second range are different from those of the first and third ranges which are identical.
The transmission disclosed in U.S. Pat. No. 4,345,488, Aug. 24, 1982 (Reed), is the most pertinent item of prior art and constitutes the immediate predecessor model as to which this invention constitutes a specific improvement. The transmission of that recent Patent is the first of its class (infinitely variable, split output, synchronously shifting, three or more range, multi-range hydromechanical steering transmission with internal steer) having a different and distinct torque ratio between the included hydrostatic component and transmission output in at least three ranges and establishes a new subclass of steering transmission. As noted in that Patent and its supporting documentation, the only previous specific recognition or teaching of progressively lower torque ratios in at least three ranges, which is the principal advantage of the subclass, occurred in "steer-over" steering transmissions (U.S. Pat. Nos. 4,164,155 and 4,164,156, Reed and Nolan, and Reed, respectively) and a single output, i.e., a truck transmission (U.S. Pat. No. 3,861,240, Nolan and Reed), although some other "steer-over" transmissions (U.S. Pat. Nos. 3,575,066; 3,596,535; and 4,258,585) could, with hindsight, be engineered to produce such results. Accordingly, the "best" and, in the sense of the foregoing classification, the only pertinent prior art is the "predecessor model" of U.S. Pat. No. 4,345,488 which uses a different and more complex gear train.